- A pain nervous pathway sometimes becomes excessively excitable; this gives rise to hyperalgesia, which means hypersensitivity to pain.  Possible causes of hyperalgesia are (1) excessive sensitivity of the pain receptors themselves, which is called primary hyperalgesia, and (2) facilitation of sensory transmission, which is called secondary hyperalgesia.
- An example of primary hyperalgesia is the extreme sensitivity of sunburned skin, which results from sensitization of the skin pain endings by local tissue products from the burn - perhaps histamine, perhaps prostaglandins, perhaps others.  Secondary hyperalgesia frequently results from lesions in the spinal cord or the thalamus.

- Is an increase in sensitivity to the pain receptors.

- After transduction of the first noxious stimuli into action potentials in the afferent neuron, a series of changes occur in components of the pain pathway - including the ion channels in the nociceptors themselves - that alter the way these components respond to subsequent stimuli, a process referred to as sensitization.  When these changes result in an increased sensitivity to painful stimuli, it is known as hyperalgesia.

Reference:
1. Guyton, Arthur C., & Hall, John E. Textbook of Medical Physiology 11th Edition. 2006.